1. Field of the Invention
This invention relates to an imaging optical system for a light beam scanning system which deflects a light bundle such as a laser beam to cause the light bundle to scan a predetermined surface.
2. Description of the Related Art
There has been known a light beam scanning system in which a light beam is deflected by a deflector such as a rotary polygonal mirror and is caused to scan a surface. Such a light beam scanning system is used, for instance, in a laser recording system or a laser read-out system. The light beam scanning system generally comprises a laser for emitting a laser beam, a rotary polygonal mirror which deflects the light bundle emitted from the laser and an imaging optical system including a f.theta. lens which focuses the deflected light bundle on a predetermined surface to be scanned to form an image in a height proportional to the deflecting angle at the rotary polygonal mirror.
In such a light beam scanning system, there has been known a surface tilt compensation optical system which corrects shift of the scanning lines due to tilt of the deflecting surface of the deflector. FIG. 10 shows an example of an imaging optical system in which the surface tilt compensation is applied. The imaging optical system comprises a deflector 50, a positive meniscus lens 52 made of plastic and an anamorphic lens 54 made of glass arranged in this order. By arranging the optical system by use of a combination of a plastic lens and a glass lens, the plastic lens may be weak in power (refracting power). Accordingly increase in the ratio of the thickness of the central portion to that of the peripheral portion can be suppressed, thereby suppressing change in optical properties of the plastic lens due to change of environment, production of the plastic lens is facilitated, the size of the optical system can be small, and the optical system can be better in its aberration and f.theta. characteristics.
In the surface tilt compensation optical system employing an anamorphic lens, the lateral magnification (magnification in the sub-scanning direction) varies depending on the scanning angle due to the anamorphic lens having power in the main scanning direction. This is mainly because the anamorphic lens must be large in the ratio of the thickness of the central portion to that of the peripheral portion in order to compensate for the surface tilt of the deflector.
FIG. 11 shows the relation between the beam diameter and the lateral magnification of an imaging optical system in which the surface tilt compensation is applied. In FIG. 11, the diameter Wo of the beam in the sub-scanning direction on the image plane (the surface to be scanned) can be expressed by the following formula. EQU Wo=M(.theta.).times.Wi
wherein Wi represents the diameter in the sub-scanning direction of a beam impinging upon the deflecting surface and M(.theta.) represents the lateral magnification as a function of the scanning angle .theta..
As can be seen from the formula above, when the lateral magnification M(.theta.) fluctuates with the scanning angle .theta., the beam diameter Wo in the sub-scanning direction fluctuates according to the scanning angle .theta. and the scanning spot cannot be uniform in diameter.